Photography by directly obtaining a positive image (a direct positive) without requiring a reversal process and a negative film is well known in the art.
Taking practical utility into consideration, conventional techniques for obtaining a positive from a direct positive silver halide photographic material, exclusive of special materials, are divided chiefly into the following two types.
One type employs a previously fogged silver halide emulsion whose fog centers (latent image) in exposed areas are destroyed making use of the solarization or Herschell effect to obtain a direct positive.
The other type uses an internal latent image type silver halide emulsion not having been fogged, which is imagewise exposed to light and then subjected to surface development either after fogging or while fogging to obtain a direct positive. The internal latent image type silver halide emulsion used herein is such an emulsion in which silver halide grains have sensitivity specs predominantly in the inside thereof and form a latent image predominantly in the inside upon exposure to light.
The methods belonging to the latter type generally enjoy higher sensitivity and are suitable for uses requiring high sensitivity as compared with the methods of the former type. The present invention belongs to the latter type.
Various techniques of this type have been proposed, such as those disclosed in U.S. Pat. Nos. 2,592,250, 2,466,957, 2,497,875, 2,588,982, 3,317,322, 3,761,266, 2,761,276, and 3,796,577, and British Patent Nos. 1,151,363, 1,140,553, and 1,011,062. According to these conventional techniques, photographic materials providing a direct positive with relatively high sensitivity can be produced.
For the details of the direct positive formation mechanism, reference can be made to it, e.g., in T. H. James, The Theory of the Photographic Process, 4th Ed., Ch. 7, pp. 182-193 and U.S. Pat. No. 3,761,276.
It is believed that a direct positive is formed through the following mechanism: First, imagewise exposure results in the formation of an internal latent image (a so-called positive hole) in the inside of silver halide grains, which leads to the formation of fog centers selectively on the surface of the unexposed silver halide grains by surface desensitization ascribed to the positive hole, and subsequent surface development results in formation of a direct positive on the unexposed area.
Selective formation of fog centers can be effected by a so-called light fog method in which the entire surface of a light-sensitive layer is secondarily exposed to light as described in British Patent No. 1,151,363 or a chemical fog method using a nucleating agent as described in Research Disclosure, Vol. 151, No. 15162 (November, 1976), pp. 76-78.
In the formation of a direct positive, the internal latent image type silver halide light-sensitive material is subjected to surface color development either after or simultaneously with fogging and then subjected to bleach and fixation (or blix). After the bleach-fixation, the material is usually washed with water and/or stabilized.
The direct positive formation by the above-described chemical fog method is disadvantageous in that the resulting image is apt to have poor graininess as compared with ordinary negatively working photographic materials. In particular, this disadvantage becomes conspicuous in cases where a color developing solution is fatigued with running the development process due to oxidation of the developing agent, reduction in pH, increase of bromine ions, etc., or in cases where the light-sensitive materials are preserved under severe conditions for a long time. Moreover, deterioration of graininess is accelerated as the pH of a developing solution is decreased. From the standpoint of graininess, therefore, it has conventionally been effective to carry out development processing at a pH of 12 or higher.
On the other hand, the rate of development is less, requiring a longer development time in the formation of a direct positive as compared with the formation of general negative images. Hence, the pH of a developing solution used in the formation of a direct positive has been increased to thereby reduce development time.
However, use of a developing solution having a higher pH value generally causes an increase in the minimum image density of the resulting direct positive. Further, under a high pH, the developing agent is more susceptible to deterioration due to air oxidation, so that development activity becomes subject to great variation.
The aforesaid light fog method does not require a high pH and, therefore, enjoys a relative practical advantage. Nevertheless, this method encounters various technical problems when applied to a broad photographic field for various purposes. That is, since this method is based on the formation of fog centers by photolysis of silver halide, the optimum illumination or exposure varies depending on the kind and characteristics of the silver halide used. It is, therefore, difficult to assure predictable performance. In addition, the development apparatus required is complicated and expensive. The rate of development is also unsatisfactory.
According to the direct positive formation by the above-described light fog method or chemical fog method, the rate of development is lower, requiring a longer development time as compared with general negatively working photographic materials. Hence, the pH and/or temperature of a development solution used in these methods may be increased to thereby reduce the development time. However, the use of a developing solution having a higher pH value, as mentioned above, generally causes an increase in the minimum image density of the resulting direct positive and the developing is more susceptible to deterioration due to air oxidation under a high pH, so that development activity becomes seriously reduced.
In addition to increasing the pH value of the developing solution, other known means for increasing the rate of development in the direct positive formation system include use of hydroquinone derivatives as disclosed in U.S. Pat. No. 3,227,552 and the use of mercapto compounds having a carboxyl group or sulfo group as disclosed in Japanese Patent Application (OPI) No. 170843/85. However, these compounds produce only small effects. Therefore, there has not yet been established an effective technique for increasing the maximum density of a direct positive without increasing the minimum density. In particular, there has been a demand for a technique for obtaining a sufficient maximum image density even when a low pH developing solution is employed.
On the other hand, the known direct positive light-sensitive materials have the disadvantage, mentioned above, that the resulting image is apt to have poor graininess as compared with ordinary negatively working photographic materials. In particular, this disadvantage becomes conspicuous in cases where a developing solution is fatigued during continuous processing due to oxidation of the developing agent, reduction in pH, increase of bromine ions, etc., or in cases where the light-sensitive materials are preserved under severe conditions for a long time. Therefore, it has been especially desired to develop a technique for obtaining sufficient graininess even when processing is carried out with a low pH developing solution.